1. Field of the Invention
The present invention generally relates to an absolute position detecting method using an encoder for detecting a position, such as a rotational position of a motor or a shift position of a table of a machine tool, and an arrangement of an encoder apparatus for executing the above method.
2. Description of the Related Art
An encoder, known as incremental encoder, has a code plate formed with numerous slits equally spaced along a circumferential periphery thereof. For one complete revolution of the code plate, attached to a rotor shaft or the like, the encoder generates a one-rotation signal and a pair of multiple-rectangular pulse signals having a predetermined phase difference therebetween. This kind of incremental encoder generally produces incremental signals on the basis of the above multiple-rectangular pulse signals independently in each of rotational directions. A counter accumulates these incremental signals to obtain a rotational shift amount (i.e. a rotational position) of the rotor in one revolution.
On the other hand, an absolute encoder has a code plate with a multi-channel pattern formed thereon as shown in FIG. 1. Using this code plate, the absolute encoder generates a code signal representing a rotational position. More specifically, as the code plate shown in FIG. 1 comprises eight channels, an eight digit binary code is outputted for expressing an absolute value of position.
In short, compared with an incremental encoder, an absolute encoder is generally complicated not only in the structure of its code plate but in the arrangement of the signal detecting and processing circuit.